The present invention relates to an upconversion fiber laser apparatus for use in displays, optical storage apparatuses, methods of processing light information, and the like.
J. Y. Allain, et al., "Blue Upconversion Fluoroziroconate Fiber Laser" Electron Lett. 26, 1990, 166 discloses the following.
When a Tm.sup.3+ ion is pumped with two red light beams having wavelengths of 674.4 nm and 647.1 nm, respectively, it emits two light beams having wavelengths of 455 nm and 480 nm, respectively. This mechanism is called an upconversion laser. The output light of 455 nm is a pulse-like beam, whereas the output light of 480 nm is a spike-like beam. They are not continuous waves (CWs).
E. W. J. Oomen, et al., "A Material and Device Study for Obtaining A Blue Upconversion Fiber Laser" Philips J. Res. 46, 157-198, 1992 discloses an analysis of the lights the Tm.sup.3+ ion emitted, and presents FIGS. 2A and 2B.
FIG. 2A shows the energy levels a Tm.sup.3+ ion takes, illustrating the energy transition the undergoes when pumped with light having wavelength of 650 nm. In FIG. 2A, the upward pointing arrows 101, 103, and 105 indicate absorptive transition (the ion absorbs light, thus acquiring energy). The downward pointing arrows 104 and 106 indicate radiative transition (the ion emits light, thus losing energy). The downward pointing, wavy line arrow 102 indicates non-radiative decay. The base level .sup.3 H.sub.6 and the other levels .sup.3 H.sub.4, .sup.3 H.sub.5, .sup.3F.sub.4, .sup.3 F.sub.2.3, .sup.1 G.sub.4, and .sup.1 D.sub.2 shall be called "level 1," "level 2," "level 3," "level 4," "level 5," "level 6," "level 7" and "level 8," respectively.
Energy exchange takes place between levels when the Tm.sup.3+ ion is pumped with light having wavelength of 650 nm. More specifically, as the Tm.sup.3+ ion absorbs the pump light, its energy shifts from the base level 1 to level 5 or 6 (arrow 101). The ion stays at level 5 or 6, but for an extremely short time. It undergoes non-radiative decay; its energy decreases to level 4 (arrow 102). The ion absorbs pump light once again, undergoing absorptive transition and, thus, shifting from level 4 to level 8 (arrow 103). The ion then transits from level 8 to level 2 (arrow 104). This transition is an radiative one, radiating blue light of 450 nm. The Tm.sup.3+ ion absorbs the pump light, transiting from level 2 to level 7 (arrow 105). The ion then undergoes radiative transition, from level 7 to the base level 1 (arrow 106). In the course of this radiative transition, the light of 480 nm is emitted.
FIG. 2B shows the results of the analysis of the light the Tm.sup.3+ ion emitted when pumped with light of 650 nm. In FIG. 2B, the population distribution at various energy levels is plotted on the ordinate, and the optical power P (W) is plotted on the abscissa. The light emitted from the thulium (Tm) ion must be continuous waves (CWs) if it is to be used in displays. In view of the physical properties of thulium, however, it is considered extremely difficult for Tm to emit CWs at wavelength of 450 nm, though Tm can emit CWs at wavelength of 480 nm.